The present invention relates to catalyst compositions and methods for their preparation and more particularly relates to a catalyst composition characterized by good activity for both oxidation and reduction reactions, and one which is particularly well suited for use as a catalyst for the purification of gases, including internal combustion engine exhaust gases.
Catalyst compositions containing a platinum group metal as a component thereof and capable of catalyzing oxidation and/or reduction reactions are of course well known in the art as shown by, for example, U.S. Pat. Nos. 3,565,830, 4,157,316 and 4,171,287, all assigned to the assignee of this application.
U.S. Pat. No. 3,565,830 discloses a catalyst composition having a platinum group metal dispersed on a film of catalytically active metal oxide, such as gamma alumina, and supported on an inert, substantially catalytically inactive refractory support. Platinum and palladium are disclosed as the preferred platinum group metals and the substantially catalytically inert support is disclosed as comprising either particulate supports, such as alumina pellets, or monolithic bodies (made of porous, refractory materials such as cordierite or of oxidation and corrosion resistant metals such as stainless steels) having a plurality of small gas flow channels passing therethrough.
U.S. Pat. No. 4,157,316 discloses a polyfunctional, so-called "three way conversion" catalyst, i.e., one capable of efficiently catalyzing the substantially simultaneous oxidation of hydro-carbons and carbon monoxide and the reduction of nitrogen oxides when an appropriate amount of oxygen is provided or maintained in the gas being treated. For example, the operation of an internal combustion engine is controlled to operate at or close to a stoichiometric proportion of air to fuel in order to maintain the oxygen content within a range in which a suitable three way conversion catalyst will promote both oxidation of unburnt hydrocarbons and carbon monoxide (to H.sub.2 O and CO.sub.2) and a reduction of nitrogen oxides (to N.sub.2). The catalyst preferably comprises platinum plus one or more additional platinum group metals selected from rhodium, ruthenium, and iridium, plus one or more oxides of the base nickel, cobalt, iron, manganese and rhenium. This patent also discloses that the alumina on which the catalytic metals are dispersed may contain stabilizing components to stabilize the aluimina coating against high temperature (1000.degree. C.-1200.degree. C.) transition to the low surface area alpha alumina form. Although ceria and mixtures of ceria and other rare earth metal oxides, particularly lanthia, are preferred stabilizers, magnesium oxide is disclosed as one such stabilizer.
As disclosed in the U.S. Pat. No. 4,056,489, also assigned to the assignee of this application, oxides of calcium, barium and strontium are among materials which may also be employed (in specified combination with rare earth metals) as stabilizers for the alumina. The stabilizers are introduced into the alumina by impregnating the alumina with water-soluble compounds of these metals and thereafter calcining the material in air at a temperature of at least 750.degree. C. to form the requisite metal oxides in the alumina lattice. As U.S. Pat. No. 3,993,572 (also assigned to the assignee of this application) shows, other techniques such as co-precipitation of alumina and the stabilizing metal from solution, may be employed.
U.S. Pat. No. 4,171,287 concerns a polyfunctional catalyst comprising a platinum group metal component and a base metal component comprising one or more base metal oxides selected from manganese; iron, cobalt, nickel and rhenium, wherein at least a major amount of these metal oxides is maintained in solid form throughout the manufacture of the catalyst composition. That is, instead of impregnating the alumina or stabilized alumina material with a water-soluble compound of the catalytic base metal and thereafter calcining the thus-impregnated alumina, (as is done with the platinum group metals) the base metal oxide component is introduced as a finely divided, solid particulate. The base metal in fine particulate form is admixed with the alumina into which the catalytic platinum group metal is, or will be, impregnated.
It is also known in the art to utilize certain metal ferrites in catalyst compositions as shown in Japanese patents: (a) 75/03987 (Jan. 16, 1975 of Kanegafuchi Chem. Industry Co., Ltd.); (b) 74/120886 (Nov. 29, 1974 of Kaneho, Ltd.); (c) 74/102590 (Sept. 27, 1974 of Research Inst. for Production Development; (d) 74/84959 (Aug. 15, 1974 of Nippon Electric Co., Ltd.); (e) 74/75461 (July 20, 1974 of Nippon Electric Co., Ltd.); (f) 74/58677 (June 6, 1974 of Research Inst. for Production Development; (g) 81/95336 (Aug. 1, 1981 of Matsushita Electric Industrial Co., Ltd.; and (h) 77/31993 (Mar. 10, 1977 of Kyoto Ceramic Co. Ltd. These patents show, respectively, the following: (a) and (b) use of copper ferrite or cobalt ferrite as a catalyst for reduction of nitrogen oxides in exhaust gases; (c) the use of manganese ferrite for carbon monoxide oxidation in exhaust gases; (d) barium ferrite for purification treatment of automobile exhaust gases; (e) manganese zinc ferrite for treatment of automobile engine exhaust gases; (f) exhaust gas purification by oxidation of carbon monoxide and hydrocarbons using ferrite catalysts; (g) the use of ferrite substances disposed on manganese-zirconium catalyst supports or platinum-palladium calcium aluminate for carbon monoxide removal from combustion heater exhaust gases; (h) the use of ferrite manganese honeycomb catalyst for automobile exhaust gas treatment.
While catalyst compositions described in the above-mentioned U.S. Patents are useful, and some have been commercially highly successful, it is of course always desirable to provide catalysts which have higher conversion efficiencies, longer life, greater resistance to catalyst poisoning and/or are obtainable at lower cost.